React better to connection state changes at runtime (e.g. client_encoding) #5146

There are some PG connection state things that we care about, and currently only read when creating a connection; for example, client_encoding determines the text encoding Npgsql uses when reading strings from PG. If a user were to change that parameter on a connection, we'd probably break badly.

At minimum, catch changes to client_encoding (and anything else?) and immediately break and throw.